1. Field of the Invention
The present invention relates generally to the field of weed control in crop plants. More particularly, it concerns the control of volunteer plants expressing a gene imparting glyphosate tolerance in a field of a glyphosate-tolerant crop species by use of tank mixtures or premixtures of a glyphosate herbicide with a second herbicide with a different mode of action, activity against the volunteer plant species, and lack of activity against the crop species. Most particularly, it relates to the control of volunteer glyphosate-tolerant corn in a crop of glyphosate-tolerant soybean by use of tank mixtures or premixtures of a glyphosate herbicide with a graminicide such as sethoxydim, clethodim, quizalofop, fluazifop, fenoxaprop, imazamox, imazethapyr, and imazaquin.
2. Description of Related Art
Chemical agents to inhibit plant metabolism have been widely used since the 1940's. One popular herbicide is glyphosate (N-phosphonomethylglycine; trade name ROUNDUP.RTM.). When applied, glyphosate is taken up by plants and subsequently inhibits 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). Inhibition of EPSPS blocks the formation of aromatic amino acids which are required components of plant hormones and vitamins. The plants in the field die after exposure to glyphosate, and any residual glyphosate in the soil is rapidly broken down. A limitation is that glyphosate may not be applied after planting of a crop, because glyphosate will cause crop injury.
By use of recombinant DNA technology, crop plants such as soybean, corn, cotton, wheat, canola, sugarbeet, rice, and lettuce, among others, have been developed with EPSPS resistant to inhibition by glyphosate. As a result, growers may use glyphosate to control weeds throughout the growing season of a glyphosate-tolerant crop without injuring crop plants.
A limitation on this technique is the presence of glyphosate-tolerant plants that are not members of the crop species. No uncultivated species of weed has been observed to naturally develop glyphosate-tolerance, and the flow of genes for glyphosate tolerance from crop plants to related wild species is not expected to occur. Rather, the expected source of glyphosate-tolerant plants that are not members of the crop species is the species planted in a given field the previous growing season. For example, in many parts of the Midwestern U.S., farmers alternate crops of corn and soybean in alternate years, a practice known as crop rotation. If kernels from the ears or whole ears of corn fall to the ground, either naturally or the result of insect damage or harvesting loss, then the fertilized kernels may lie dormant over the winter and germinate during the following growing season. Corn plants growing in a crop of soybeans, the corn plants being the progeny of a crop of corn grown in that field the previous growing season, are called volunteer corn plants. More generally, volunteer plants of any species may grow in situations in which farmers practice crop rotation between any two species.
Volunteer plants are as undesirable as undomesticated weeds, in that both grow by absorbing sunlight, soil nutrients, and soil moisture that the farmer prefers be absorbed by the crop species. Previously, volunteer plants, like weeds, could be controlled by application of a broad spectrum herbicide such as glyphosate. However, when the volunteer plants are progeny of a glyphosate-tolerant crop, the application of glyphosate to the field will control undomesticated weed species, but not the volunteer plant species. Given the wide adoption of glyphosate-tolerant species, and the practice of crop rotation, a need exists for a method for controlling glyphosate-tolerant volunteer plant species growing in a field of a glyphosate-tolerant crop species.
Therefore, a method to allow convenient control of both glyphosate-tolerant volunteer plant species and glyphosate-susceptible weeds by a single application is desirable. It is also desirable for such a method to control volunteer plants and weeds without injury to or yield loss in the crop species.